Preparation of selenide phosphors



United States Patent Gilmore E. Crosby, Millersville, Pm, .assignor toRadio Corporation of America, a corporation of Delaware No Drawing.Application 0ctober31, 1956 Serial No. 619,395

11 Claims. (Cl. 252-301.6)

This is a continuation-impart of patent application Serial No. 349,553,filed April 17, 1953, by Gilmore E. Crosby, and now abandoned.

This invention relates to improved methods of preparing zinc selenide,zinc-cadmium selenide, and cadmium selenide; and particularly to methodsof preparing selenides which are substantially free of sulfides andwhich do not require hydrogen selenide as an intermediate.

Zinc selenide and zinc-cadmium selenide are important phosphorcomponents and, activated with copper, are important phosphors for usein kinescopes employed in color television receivers and forelectroluminescent devices. In order to prepare zinc selenide andzinc-cadmium selenide phosphors having high luminescence efiiciency,good stability, and desirable emission spectra, it is necessary to startwith selenides of high quality. Zinc selenide is also used to coatlenses of optical devices and as an intermediate in preparing hydrogenselenide. Cadmium selenide is an important raw material from whichphotoconducting and conducting powders can be made. For commercialproduction, it is also desirable to employ a method of preparing theselenides which is economical in cost.

One previously proposed method for preparing zinc selenide is to passhydrogen selenide gas through a solution of the metal salt. Anothermethod is to combine the vapor of free zinc and free selenium in areaction tube at elevated temperatures. The latter reaction has thedisadvantage of being explosive and losses can occur by vaporization ofunreacted portions of the elements. Furthermore, hydrogen selenide gasis poisonous and difficult to handle. It is therefore desirable to avoidits use in commercial preparations. Another method is to fire a drymixture of 2 moles of zinc oxide, 1 mole of zinc sulfide and 3 moles ofselenium to obtain the reaction 2ZnO+ZnS+3Se 3ZnSe-l-SO It has beenfound that considerable amounts of Se and SeO are volatilized from thisprocess and that the reaction product is contaminated with sulfides. Ithas also been proposed to substitute cadmium for part or all of the zincin the foregoing processes.

One object of the present invention is to provide improved methods ofpreparing high quality zinc selenide, zinc cadmium selenide, or cadmiumselenide.

Another object of the present invention is to provide safe,non-explosive, dry methods of preparing zinc selenide, zinc cadmiumselenide or cadnium selenide.

Still another object of the invention is to provide improved methods ofpreparing zinc selenide or zinc cadmium selenide capable of being madeinto efficient phosphors.

Another object of the invention is to provide improved methods ofpreparing cadmium selenide capable of being made into photoconductingand conducting powders.

A further object of the present invention is to provide improved methodsof making zinc selenide, zinc cadmium selenide or cadmium selenide ofhigh purity and which is substantially free of sulfides.

2,818,391 Patented Dec. 31, 1957 These and other objects will be moreapparent and the invention will be more readily understood from thespecific examples and the detailed description which follows.

In general, the present invention comprises an improved method ofpreparing zinc selenide, zinc cadmium selenide or cadmium selenide byfiring a mixture of selenium, at least one oxide selected from the groupconsisting of zinc oxide and cadmium oxide and at least one sulfideselected from the group consisting of zinc sulfide and cadmium sulfideat a temperature above 600 C., the ratio of moles of selenium to molesof sulfide being about 3 to 0.9 and the ratio of moles of oxide to molesof sulfide being about 2.75 to 0.9.

Example 1.--A red-emitting zinc selenide phosphor can be prepared asfollows. A mixture is prepared consisting of 3 gm. moles of powderedselenium, 2.75 gm. moles of zinc oxide, .9 gm. moles of Zinc sulfide,8.4 grns. of lithium bromide and .06 gm. of cupric chloride. Thismixture is thoroughly mixed until it is homogeneous throughout and isthen fired in a covered quartz crucible at a temperature of about 920 C.for 1 hour.

After cooling, it is preferable to wash out any unreacted ingredients.Unreacted zinc oxide can be washed out by treating the product withdilute hydrochloric acid, acetic acid, sulfuric acid or other acid inwhich zinc oxide is soluble. Excess acid must then be washed out withwater. Unreacted selenium may be removed by treating the product with a10% solution of potassium cyanide. Sodium sulfite solution may also beused to remove unreacted selenium.

The above material, which contains a small amount of copper activator,is luminescent and emits in the red region when excited with cathoderays. If a phosphor is not desired, the copper salt and flux areomitted.

In making a phosphor by the above-described method it is preferable touse very pure starting materials. The selenium should first be purifiedby distillation and the zinc oxide and zinc sulfide should bespectroscopically pure.

The efiect of more than theoretical zinc 0xide.-As indicated above, anexcess of zinc oxide with relation to selenium is preferred. Although itis not absolutely necessary, it is preferred to have no unreactedselenium in the product. However, the product may be treated asindicated in Example 1 to remove unreacted zinc oxide or selenium.

There is a beneficial efiect in using excess zinc oxide in theformulation. To show the effect of excess zinc oxide, samples wereprepared according to the procedure of Example 1 but in place of the molratio of ZnO/ZnS/Se of 2.75/ 0.9/ 3.0, the mol proportion of zinc oxidewas varied from 2.0 to 3.0, 2.0 being the theoretical quantity required.Table I shows the results of this investigation. The peak efficienciesgiven are the average of three series, the loss in firing are theaverage of two series. The color remained at 6400 A. for all samples. Itis seen that the optimum efiiciency is obtained with a mol ratio of2.75/0.9/3.0. Also there is less loss by volatilization during firing asthe proportion zinc oxide is increased.

In the dry process reaction, the batch will lose weight during firing. Alarge portion of this weight loss will be the by-product S0 The otherportion of the weight loss will be SeO Since Zinc sulfide is used inless than theoretical quantities, SeO will be present in excess and'thuswill evolve. Se0 has a sublimation temperature of 317 C. The SeO alongwith the S0 constitute the theoretical weight loss during firing.Actually in a batch which will theoretically yield 353.4 grams of ZnSe,the theoretical loss by evolution will be 58.1 grams, 19.2% of whichwill be SeO Table I shows that the volatile loss greater thantheoretical is less with, high zincoxide concentrations. The loss byvolatilization can also be reduced by increasing the batch size.

The eficct of less than theoretical zinc sulfide.Zinc sulfide is used inless than theoretical amounts so that sulfide is eliminated from thefinal product. Table ll shows the effect of using less than theoreticaland stoichiometric quantities. The samples represented in Table itcontain a 3.0 to 3.0 ZnO to Se ratio and 3% CdSe in the final product.The CdSe was incorporated by substituting some CdS for ZnS. For thepurpose of Table ll, CdS was changed mole for mole to ZnS. As can beseen in Table ii, the luminescent color peaks between 6650 A. and 6660A. for 0.9 mol ZnS. This is the same color obtained from a phosphorprepared from precipitated zinc cadmium selenide. The table shows thatthe color is shifted toward the yellow when theoretical quantities ofZnS are used. Samples having the correct color were analyzed and foundto have 0.5% sulfur. This shows that even with a deficiency of zincsulfide as a reactant some sulfur remains in the product but is notsutiicient to shift the luminescent color.

Effect of firing atmosphere on the final phosphr.To study the efiect offiring atmospheres on the phosphor efficiency, samples were prepared asoutlined in Example I but the firing conditions were changed. Firing wasperformed by placing the sample in a quartz boat, inserting the filledboat in a quartz tube and placing the quartz tube with sample in afurnace at 900 C. for one hour. The atmosphere in the quartz tube wascontrolled thus permitting different atmospheres for each sample.Samples were fired in atmospheres as listed in Table Hi. There was noshift in luminescent color. Table HI shows the varying peak efficienciesresulting from various atmospheres. It will be noticed that highlyoxidizing or reducing atmospheres give valueless products. Neutral orslightly oxidizing atmospheres are beneficial. it must be rememberedthat a by-product of the dry process reaction is S0 Therefore firing inN is not a totally neutral atmosphere. The best atmosphere is CO withatmospheres of S0 and N being acceptable.

TABLE III Efficiency of phosphors prepared under varying atmospheresRelative Atmosphere Peak Elficieney SOs 101 C 02 C O 37 N 2 98 NPRinert;

Fr 28 Air inert Oz inert Discussion of the firing procedure.-One firingtechnique comprises loading the material to be fired in a quartzcrucible, placing an inverted quartz lid on top of the material, fillingthe inverted lid with unfired zinc sulfide, covering the entire cruciblewith a quartz lid, and firing. The quartz lid filled with zinc sulfidegives a simple method of maintaining an S0 atmosphere. The zinc sulfidereacts with any air present to give S0 and ZnO. From the study onatmospheres it is seen that an $0 atmosphere is desirable. The dryprocess yields S0 as a by-product and, it would seem, should be capableof providing its own atmosphere. This is the case but the inverted lidcontaining zinc sulfide on top of the charge gave slightly improvedproducts.

From the previous section it is obvious that a C0 at mosphere is mostpreferable. This type atmosphere should be used if a simple factorytechnique of giving a C0 atmosphere can be provided.

Discussion of the washing procedare.-Washing the fired product isdesirable. Adequate washing procedure removes unreacted zinc oxide andtraces of elemental selenium. The need for excess zinc oxide has beenshown. This excess is removed after firing so that better eflicienciescan be obtained. The excess zinc oxide may be removed by addinghydrochloric acid to a water slurry of the fired material until the pHis lowered and remains at 2.0. By acidifying to pH 2.0, the excess zincoxide is reacted, but the acidity is not great enough to causedecomposition of the zinc selenide at room temperature.

The dry process zinc selenide contains some elemental selenium. This maybe removed 'by washing in a 10% sodium sulfite solution. A hot solutionof sodium sulfite will give a rapid and eificient dissolution ofselenium, but a cold solution is successful only if sufficient time isgiven for traces of selenium to dissolve. While sodium sulfite isrecommended for factory processing, a solution of sodium or potassiumcyanide is also effective in removing selenium.

The efiect of firing time and temperature.-The firing temperature may bevaried considerably. It is preferably kept within a range of about 600C. to about 1000 C. but this is not critical. If the temperature is toohigh, however, some of the selenium boils away and is lost.

The time of firing also is not critical. In general, it may be variedfrom about 15 minutes to several hours. The temperature of firing andthe size of the batch are factors which largely control the optimumlength of firing time.

Results of firing batches for varying times at various temperatures aregiven in Table IV. It will be noted that the average particle sizes aregiven for batches with good efficiencies, the particle size beingimportant in application work. Actually a compromise between optimumfiring temperature and time and the resulting particle size has to bemade. The smallest possible particle size is desired yet a reasonablefiring time is important. The firing temperatures and times such as 700C. for /2 hour and then 900 C. for 1 hour, gave an efficient 5 product(100% Peak Eff. Part. size approx; 32 1),. Firing at 700 C. for thefirst /2 hour reduces lossuby volatiliza tion and gives a more desirableyield.

TABLE IV Firing time and temperature versus efiiciency' and particlesize l hour 1 hour 2 hours 3'hours Firing Time, Firing. Peak ParticlePeak Particle Peak- Particle Peak Particle Temp. Efi. Size, Eff. Size,'Efi. Size; Efl. Size,

Microns Mierons Flux and flux c0ncentrati0n.Although it is desirable touse a flux when a phosphor is'being prepared, the flux may be omittedentirely. It is also omitted if non-luminescent zinc selenide is beingprepared or if cadmium compounds are substituted and cadmium selenide isbeing prepared. Suitable fluxes are any of the alkali. halides includingthe ammonium halides, also the alkaline earth metal halides.Luminescence efficiency of the phosphor has been found to vary with theparticular halide used. The preferred amount of flux is about 2% byweight of the selenide product but this is not at all. critical. Amountsup to can be used, for example. Samples of the zinc selenide phosphorwere made according to Example 1, except the flux and flux concentrationwere varied. Thirteen different fluxes were used at concentrations of 1,3, 6.and 10% of the total weight of starting materials. Each flux wasadded as solution having been previously purified by slurrying withprecipitated ZnS and filtering. The evalution of thepeak and visualefliciencies together with the luminescent peak. wavelength indicatedthe most desirable flux and flux concentration. Peak etficienciesresulting from theuse of difierent fluxes at varying concentrations aregiven in Table V. The average efiiciencies and color are given in TableVI. Lithium bromide was selected as giving optimum results althoughcalcium bromide, strontium chloride and strontium bromide were alsoacceptable. The optimum flux concentration was between 1 and 3% of thetotal'weight of starting materials.

TABLE V Peak efficiencies of ZnSe ph sphors using difierent fluxes atvarying concentrations TABLE .VI

Average eyficzencies and color resulting from varying fluxes AverageAverage Average Flux Peak Visual Peak E11. E11. Lum. Color- 101 104*6465 6450 68 as 6410 97 100 51 73 6413 74 83; 6405 93 01 6430 49 70 6360100 me 6425 105 116 6415 104 105 6430 104 106 0410 94 6430 84 99-= 643DOptimum activator concentration.The amount ofi copper activator in theproduct may be varied between about .001 and 0.2% by Weight based on theweight of the selenide product. The preferred range is about .005 to.008% and the amount of copper chloride given in the example is intendedto furnish about .0075 copper in the product. To determine the optimumactivator concentration, samples Were made according to the procedure ofExample 1, except the activator concentration was varied from .001% to020% by weight of'the theoretical yield. The luminescent results aregiven in Table VII. It will be noted that maximum efiiciencies areobtained with an activator concentration between. .005.% and 010% andthat the luminescent color shifts toward the yellow with increasingactivator concentration. An

activator concentration of .0075% was selected for the standardprocedure.

TABLE VII Efiiciencies and color versus activat r cone Relative RelativePeak Percent Cu Activator Peakv Visual Lum.

Efi. Efl. Color A If it is desired to prepare zinc cadmium selenideinstead of zinc selenide, part of the zinc in the raw batch can bereplaced mol for mol with cadmium. If it is desired to prepare cadmiumselenide, cadmium is substituted for all of the Zinc in the raw batch.

The mechanism of the reaction is considered to be as follows: Theselenium and zinc oxide or cadmium oxide react to give zinc selenide orcadmium selenide and selenium dioxide. The selenium dioxide reactsimmediately with the zinc sulfide or cadmium sulfide to give additionalzincselenide or cadmium selenide and sulfur dioxide gas. The gas isdriven off during the firing operation.

The process of the invention has been found to be suitable for theproduction of zinc selenide, zinccadmium selenide or cadmium selenide incommercial quantities and has been found particularly advantageous forthe production of pure phosphor materials. In preparing phosphors,activators other than copper may, of course, be used in any conventionalamount.

Example 2.Pure zinc selenide may be prepared as follows. A mixture isprepared consisting of 3 gm. moles of powdered selenium, 2.75 gm. molesof zinc oxide, and 9 gm. moles of ZnS. This mixture is thoroughly mixeduntil homogeneous throughout and is then fired in a covered crucible ata temperature of about 800 C. for 1 hour. The product may be washed asin Example 1.

Example 3.Pure zinc-cadmium selenide may be prepared according to theprocedure of Example 2 except the raw batch is:

2.75 gm. moles ZnO 0.6 gm. moles ZnS 0.3 gm. moles CdS 3.0 gm. moles SeThe product has the approximate composition 0.9 ZnSe:0.1 CdSe.

Example 4.Pure zinc cadmium selenide may be prepared according to theprocedure of Example 2 except that the raw batch is:

1.90 gm. moles ZnO .85 gm. moles CdO 0.9 gm. moles CdS 3.0 gm. moles SeThe product has the approximate composition 0.4 ZnSe:0.6 CdSe.

Example 5.Pure cadmium selenide may be prepared as follows. A mixture isprepared consisting of 3.0 gm. moles of powdered selenium, 2.75 gm.moles of cadmium oxide, and 0.9 gm. moles cadmium sulfide. This mixtureis thoroughly mixed until homogeneous throughout and is then fired in acovered crucible at a temperature of about 700 C. for 1 hour. To removeexcess CdO, the temperature can be raised to above 950 C. The productmay be washed as in Example 1.

Example 6.Cadm.ium selenide may be prepared as follows. A mixture isprepared consisting of 3.0 gram moles of powdered selenium, 2.75 grammoles of cadmium oxide (or cadmium carbonate), and 0.9 gram molescadmium sulfide. For some uses, it may be desirable to include an excessof selenium over 3.0 gram moles. This mixture is thoroughly mixed untilhomogeneous throughout and is then fired in a covered crucible at atemperature of about 700 C. for 1 hour. The product may be Washed as inExample 1.

Example 7.Photoconducting cadmium selenide may be prepared as follows. Amixture is prepared consisting of 3.0 gram moles of powdered selenium,2.75 gram moles of cadmium carbonate and 0.9 gram moles cadmium sulfide,42 grams cadmium chloride, and .085 gram cupric chloride. This mixtureis thoroughly mixed until homogeneous throughout and is then fired in acovered crucible at a temperature of about 700 C. for 1 hour. Theproduct may be washed as in Example 1.

Example 8.Electrolurninescent Zinc selenide may be prepared as follows.A mixture is prepared consisting of 3.0 gram moles of powdered selenium,2.75 gram moles of zinc oxide, 0.9 gram moles of zinc sulfide, 12 gramslithium bromide, and 0.85 gram cupric bromide. This mixture isthoroughly mixed until homogeneous throughout and is fired in a coveredcrucible at a temperature of about 700 C. for /2 hour and then at atemperature of about 1050" C. for 1 hours. The product may be washed asin Example 1.

What is claimed is:

1. A method of preparing a material selected from the group consistingof zinc selenide, zinc-cadmium selenide and cadmium selenide comprisingfiring a mixture of selenium, at least one oxide selected from the groupconsisting of zinc oxide and cadmium oxide and at least one sulfideselected from the group consisting of zinc sulfide and cadmium sulfideat a temperature of 600 C. to

1050 C., the ratio of moles of selenium to moles of sulfide being about3 to 0.9 and the ratio of moles of oxide to moles of sulfide being about2.75 to 0.9.

2. The method of claim 1 in which said reaction is carried out at atemperature of about 700 C. to 1000 C.

3. A method of preparing zinc selenide comprising firing a mixture ofselenium, Zinc oxide and zinc sulfide at a temperature between about 700C. and 1000 C., the ratio of moles of selenium to moles of zinc sulfidebeing about 3 to 0.9 and the ratio of moles of zinc oxide to moles ofzinc sulfide being about 2.75 to 0.9.

4. The method of claim 3 in which cadmium is substituted for up to ofthe zinc.

5. A method of preparing zinc selenide comprising firing a mixture ofselenium, zinc oxide and zinc sulfide at a temperature of about 920 C.,the ratio of moles of selenium to moles of zinc sulfide being about 3 to0.9 and the ratio of moles of zinc oxide to moles of zinc sulfide beingabout 2.75 to 0.9.

6. A method of preparing cadmium selenide comprising firing a mixture ofselenium, cadmium oxide and cadmium sulfide at a temperature betweenabout 600 C. and 1000 C., the ratio of moles of selenium to moles ofcadmium sulfide being about 3 to 0.9 and the ratio of moles of cadmiumoxide to moles of cadmium sulfide being about 2.75 to 0.9.

7. A method of preparing a phosphor selected from the group consistingof copper-activated zinc selenide and copper-activated zinc-cadmiumselenide comprising reacting a mixture of selenium, at least one oxideselected from the group consisting of Zinc oxide and cadmium oxide, acopper activator and at least one sulfide selected from the groupconsisting of zinc sulfide and cadmium sulfide at a temperature of 600C. to 1050 C., the ratio of moles of selenium to moles of sulfide beingabout 3 to 0.9, the ratio of moles of oxide to moles of sulfide beingabout 2.75 to 0.9.

8. A method of preparing a copper-activated zinc selenide phosphorcomprising firing a mixture of selenium, Zinc oxide, zinc sulfide and acopper activator at a temperature between 700 C. and 1000 C., the ratioof moles of selenium to moles of zinc sulfide being about 3.0 to 0.9 andthe ratio of moles of zinc oxide to moles of zinc sulfide being about2.75 to 0.9.

9. A method of preparing a photoconducting material comprising firing amixture of selenium, cadmium oxide, cadmium sulfide and a copperactivator at a temperature between 600 C. and 1000 C., the ratio ofmoles of selenium to moles of cadmium sulfide being about 3.0 to 0.9 andthe ratio of moles of cadmium oxide to moles of cadmium sulfide beingabout 2.75 to 0.9.

10. The method of claim 9 wherein said temperature is about 700 C.

11. A method for preparing an electroluminescent phosphor comprisingfiring a mixture of selenium, zinc oxide, zinc sulfide, a copperactivator and a flux at a temperature of about 700 C., the ratio ofmoles of zinc oxide to zinc sulfide to selenium being about 2.5 to 0.9to 3.0; and then further firing the mixture at about 1050 C.

References Cited in the file of this patent UNITED STATES PATENTS2,546,239 Rothschild Mar. 27, 1951 FOREIGN PATENTS 699,302 Germany Nov.26, 1940 column 8,, line 58, for "about 2.5" read am about 2375 o UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION.

Patent No. 2,818,391 December 31, 1957 Gilmore ED Crosby It is herebycertified that error appears in the printed specification ef the abovenumbered patent requiring correction and. that the said Letters Patentshould read as corrected below.

Column 6, line '74, for "9 gm. moles" read =1--==- ,9 gmn moles Signedand sealed this 18th day of February 19580 (SEAL) Attest KARL Ho.AXLIIE' I ROBERT Co WATSON Attesting Officer Commissioner of Patents

1.A METHOD OF PREPARING A MATERIAL SELECTED FROM THE GROUP CONSISTING OFZINC SELENIDE, ZINC-CADMIUM SELENIDE AND CADMIUM SELENIDE COMPRISINGFRINGIN A MIXTURE OF SELENIUM, AT LEAST ONE OXIDE SELECTED FROM GROUPCONSISTING OF ZINC OXIDE AND CADMIUM OXIDE AND AT LEAST ONE SULFIDESELECTED FROM THE GROUP CONSISTING OF ZINC SULFIDE AND CADMIUM SULFIDEAT A TEMPERATURE OF 600*C. TO 1050*C., THE RATIO OF MOLES OF SELENIUM TOMOLES OF SULFIDE BEING ABOUT 3 TO 0.9 AND THE RATIO OF MOLES OF OXIDE TOMOLES OF SULFIDE BEING ABOUT 2.75 TO 0.9.